Fuel assemblies for nuclear reactors are generally provided in the form of fuel elements or rod arrays maintained by a structure including welded spacer grids, a lower end fitting, and an upper end fitting. Guide tubes provide the structural integrity between the lower end fitting, the upper end fitting and the grid spacers intermediate the ends. The grid spacers define an array of fuel rods which typically may be rows and columns of 16 rods each. One such support grid is disclosed in U.S. Pat. No. 3,423,287.
The typical fuel element support grid for supporting an array of nuclear fuel elements or rods intermediate their ends includes a generally quadrangular perimeter. A plurality of fuel element compartments within the perimeter are defined by first and second slottedly interlocked grid-forming members welded to the perimeter and joined to each other by small nugget welds at the ends of their lines of intersection.
The grid-forming members of the fuel element support grid are each slotted for one-half of their width along their lines of intersection with the other grid-forming members of the array such that they may be assembled and interlocked at the lines of intersection in what may be termed "egg-crate" fashion. This structure has been utilized in that it provides a good strength to weight ratio without severely affecting the flow of cooling or moderating fluids through the grid in the nuclear reactor. The grid-forming members typically include projecting springs and arches for engaging and supporting the fuel elements within the compartments. At each fuel rod grid position in the fuel assembly, axial, lateral and rotational restraint is provided against fuel rod motion during operation with fuel coolant flow or during seismic disturbance or external impact. These spacer grids also act as lateral guides during insertion and withdrawl of the fuel assembly from the reactor. All of the elements of the fuel lattice, including the leaf springs and the arches within the compartments, are parallel to the fuel coolant flow to minimize pressure drop across the grid.